1. Field of the Invention
The present invention relates generally to a method of driving a simple matrix type liquid crystal display panel using STN liquid crystals or the like and, more particularly, to a method of driving a liquid crystal display panel of low power consumption suitable for intermediate tone display by frame modulation.
2. Description of the Related Art
A simple matrix type liquid crystal panel is constructed by maintaining a liquid crystal layer between a row electrode group and a column electrode group to define a plurality of pixels in matrix form. Further, as methods for driving the simple matrix type liquid crystal display panel, there are the voltage averaging method, the SA method and the MLA method.
The voltage averaging method is a method of driving a simple matrix type liquid crystal display panel for successively selecting respective row electrodes one by one and providing all the column electrodes with data signals in correspondence with ON/OFF states in accordance with selected timings. Therefore, the voltage applied to respective electrodes becomes high only once in one frame cycle T for selecting all the row electrodes and becomes a constant bias voltage during a remaining nonselection time period. According to the voltage averaging method, when the response speed of the liquid crystal material used is slow, there is provided a change in brightness in accordance with the effective value of the waveform of the applied voltage in the one frame cycle to thereby maintain the most suitable contrast for the conditions. However, when the division number is increased and frame frequency is reduced, the difference between frame cycle time and response time of liquid crystal is reduced, the liquid crystal responds separately to each applied pulse, a flicker in brightness referred to as a frame response phenomenon becomes apparent, and the contrast is reduced.
The SA (Smart Addressing) method is another method for driving a simple matrix type liquid crystal display panel. In either the voltage averaging method or the SA method, the respective row electrodes are successively selected one at a time and data signals in correspondence with ON/OFF states are provided to all the column electrodes in conformity with selected timings. However, common nonselection levels of contiguous frames are different from each other in the former and the same in the latter.
The MLA method is also referred to as the multiple line addressing or multiple line selecting method for simultaneously selecting a plurality of row electrodes so that apparent high frequency formation is achieved and the frame response phenomenon which is problematic in the voltage averaging method is restrained. A unique scheme is adopted in the MLA method in order to simultaneously select a plurality of row electrodes and display respective pixels independently from each other. In this scheme there is carried out set successive scanning involving applying a plurality of row signals represented by a set of orthogonal functions to a row electrode group according to a set order for each respective selection time, there is successively carried out a cross-products operation between the set of orthogonal functions and a set of selected pixel data, and column signals having voltage levels in accordance with the result of the cross-products operation are applied to a column electrode group during the selection time in synchronism with the successive scanning of the set.
Further, the MLA method is disclosed in Japanese Patent Laid Open No. 100642/1993, Japanese Patent Laid-Open No. 27907/1994, Japanese Patent Laid-Open No. 72454/1995, Japanese Patent Laid-Open No. 193679/1995, Japanese Patent Laid-Open No. 199863/1995, Japanese Patent Laid-Open No. 311564/1995, Japanese Patent Laid-Open No. 184807/1996, Japanese Patent Laid-Open No. 184808/1996, Japanese Patent Laid-Open No. 2000-19482 and so on.
Next, as multiple gradation display methods of a simple matrix type liquid crystal display panel, there are generally provided a pulse width modulating system and a frame modulating system, with the latter established as an inexpensive, technologically sound method. The frame modulating system is a system in which two gradations of ON/OFF are selectively chosen over a plurality of frames to thereby provide two or more gradations utilizing temporal average values. Further, the intermediate tone display of the simple matrix type liquid crystal display panel is realized by a combination of the driving method and the multiple gradation display method.
Here, an investigation will be given of power consumption of a simple matrix type liquid crystal display panel with the frame modulating system as the multiple gradation display method, comparing when the panel is driven respectively by the voltage averaging method, the SA method and the MLA method. Further, the frame modulation is carried out by either one row at a time or one pixel at a time.
FIG. 2 shows an example of a 5 gradation frame modulation pattern applied to a simple matrix type liquid crystal display panel. In FIG. 2, at gradation level 0, all values of intersections of rows and columns of the simple matrix type liquid crystal display panel are represented by 0 (OFF) from the 1-st frame to the 4-th frame. Here, the simple matrix type liquid crystal display panel is provided with a matrix of N rowsxc3x97M columns.
At gradation level 1, 1 (ON) is given to pixels at intersections of (2n+1)-th row and odd number columns of the 1st frame, intersections of (2n+1)-th row and even number columns of the 2nd frame, intersections of (2n+2)-th row and odd number columns of the 3rd frame and intersections of (2n+2)-th row and even number columns of the 4th frame and 0 (OFF) is given to other pixels in the simple matrix type liquid crystal display panel. Here, notation n designates an integer of 0 through N/2. Therefore, notation (2n+1) throw represents an odd number row and notation (2n+2)-th row represents an even number row contiguous thereto.
At gradation level 2, 1 (ON) is given to pixels at intersections of the (2n+1)-th row and odd number rows of the 1st frame, intersections of the (2n+2)-th row and even number columns of the 1st frame, intersections of the (2n+1)-th row and even number columns of the 2nd frame, intersections of the (2n+1)-th row and odd number columns of the 3rd frame, intersections of (2n+2)-th row and even number columns of the 3rd frame, intersections of the (2n+1)-th row and odd number columns of the 4th frame and intersections of the (2n+2)-th row and even number columns of 4-th frame, and 0 (OFF) is given to other pixels of the simple matrix type liquid crystal display panel.
At gradation level 3, 0 (OFF) is given to pixels at intersections of the (2n+1)-th row and odd number columns of the 1st frame, intersections of the (2n+1)-throw and even number columns of the 2nd frame, intersections of the (2n+2)-th row and odd number columns of the 3rd frame and intersections of the (2n+2)-th row and even number columns of the 4th frame, and 1 (ON) is given to other pixels in the simple matrix type liquid crystal display panel.
At gradation level 4, 1 (ON) is given to all of the pixels at intersections of rows and columns of the simple matrix type liquid crystal display panel from 1-th frame to 4-th frame.
First, FIGS. 5A and 5B show column electrode waveforms when multiple gradation display is carried out by applying the frame modulating system based on the 5 gradation frame modulation pattern of FIG. 2 to the simple matrix type liquid crystal display panel driven by the voltage averaging method or the SA method and when scanning is carried out from an upper portion to a lower portion of a screen. However, for simplifying the explanation, display data is data of one color of intermediate tone.
That is, FIG. 5A shows a column electrode waveform when the pixels of the intersections of a certain column""s electrodes with those of the (2n+1)-th row and of the (2n+2)-th row for any applicable n are both ON or both OFF in the 5 gradation frame modulation pattern of FIG. 2, this waveform being indicated by hatched lines. The level of the column electrode waveform in this case is +1√N in selection time t in one frame period T and xe2x88x921√N in remaining nonselection time (Txe2x88x92t). At the next frame, the level is inverted and there is a similar column voltage waveform. Therefore, when both upper and lower rows are made ON or OFF at an intermediate gradation level, the number of changes of the column electrode waveform in one frame is 1.
Further, FIG. 5B shows a column electrode waveform when one of pixels at respective intersections of a certain column electrode and the (2n+1)-th row electrode and the (2n+2)-th row electrode for any applicable n is made ON and other is made OFF in the 5 gradation frame modulation pattern of FIG. 2, this waveform indicated by hatched lines. The level of the column electrode waveform in this case is +1√N in selecting time t of one frame period T. In the remaining nonselection time (Txe2x88x92t), at the initial t-length period the level is xe2x88x921√N, at the next t-length period the level is +1√N and so on thereafter, the level is similarly changed until the final t. At successive frames, the level is inverted and a similar column voltage waveform is shown. Therefore, when the pixel is made ON and OFF at every other row at the intermediate gradation level, the number of changes of the column electrode waveform in one frame is N, the same as the number of row electrodes.
Next, FIGS. 7A and 7B show column electrode waveforms when multiple gradation display is carried out by applying the frame modulation system based on the 5 gradation frame modulation pattern of FIG. 2 to the simple matrix type liquid crystal display panel driven by the MLA method and when scanning is carried out successively from an upper portion to a lower portion of a screen. Further, for simplifying the explanation, displayed data is data of one color of intermediate tone.
Meanwhile, there are a nondistributed type and a distributed type in the MLA driving method. According to the nondistributed type MLA driving method, row function voltage given by an orthogonal function table is applied to a plurality of row electrodes simultaneously selected, and not applied evenly throughout one frame period. In contrast thereto, according to the distributed type MLA driving method, row function voltage given by an orthogonal function table is applied to a plurality of row electrodes simultaneously selected and applied evenly throughout one frame period.
Explaining the nondistributed type MLA driving method in reference to an orthogonal function table of FIG. 3, in first selection time t, voltages 1, xe2x88x921, xe2x88x921, and xe2x88x921 (relative values) are respectively applied to four electrodes of the (2n+1)-th row, the (2n+2)-th row, the (2n+3)-th row and the (2n+4)-th row. The same four row electrodes are applied with voltages xe2x88x921, 1, xe2x88x921 and xe2x88x921 at the second selection time t, voltages xe2x88x921, xe2x88x921, 1 and xe2x88x921 at third selection time t and voltages xe2x88x921, xe2x88x921, xe2x88x921 and 1 at successive fourth selection time t, respectively, In this way, the row function voltages given by the orthogonal function table are applied to the plurality of row electrodes simultaneously selected without being distributed. Therefore, in the case of the nondistributed type MLA method, for simultaneously selecting four voltages by using the orthogonal function table of FIG. 3, selection time is 4t and nonselection time is (Txe2x88x924t).
FIG. 7A shows a column electrode waveform when the two pixels at the intersections of a certain column electrode and the (2n+1)-th row electrode and the (2n+2)-th row electrode for any applicable n are made both ON or both OFF, this waveform indicated by hatched lines. In selection time 4t of one frame period T the level of the column electrode waveform in this case is +2√N at initial t, xe2x88x922√N at the next 3t and xe2x88x922√N at remaining nonselection time (Txe2x88x924t). The level is inverted at a successive frame and similar column voltage waveform is shown. Therefore, when both of upper and lower rows are made ON or OFF at intermediate gradation level, a number of changes of the column electrode waveform in one frame is 1.
FIG. 7B shows a column electrode waveform when one of the pixels at respective intersections of a certain column electrode and the (2n+1)-th row electrode and the (2n+2)-th row electrode is made ON and other is made OFF in the 5 gradation frame modulation pattern of FIG. 2, this waveform indicated by hatched lines. In selection time 4t of one frame period T, the level of the column electrode waveform in this case is +2√N at initial t, and xe2x88x922√N at the next 3t. In remaining nonselecting time (Txe2x88x924t), the level is xe2x88x922√N at initial 4t, +2√N at the next 4t and thereafter, the level is similarly changed repeatedly until final 4t. At the next frame, the level is inverted and similar column voltage waveform is shown. Therefore, when the column is made ON and OFF at every other piece at an intermediate gradation level, the number of changes of the column electrode waveform in one frame is N/8.
Further, even when the panel is driven by the distributed type MLA method, the number of changes of the column electrode waveform in one frame is 1 when both upper and lower columns are made ON or OFF at intermediate gradation level and N/8 when the column voltage is changed between ON and OFF at every other electrode at intermediate gradation level.
Meanwhile, power consumption of a liquid crystal panel is determined by free discharge current between row electrodes and column electrodes. In other words, power consumption of a liquid crystal panel is determined by values of voltages between row electrodes and column electrodes and waveform (amount of change).
However, in the simple matrix type liquid crystal panel for carrying out multiple gradation display by the frame modulating system, when the panel is respectively driven by the voltage average method, the SA method or the MLA method and the screen is scanned successively from the upper portion to the lower portion, the column electrode waveform is changed a large number of times, N times in the case of FIG. 5B or N/8 times in the case of FIG. 7B in one frame. That is, according to the conventional scanning system of scanning successively the screen from the upper portion to the lower portion, in the simple matrix type liquid crystal panel driven by the voltage averaging method, the SA method or the MLA method and carrying out the multiple gradation display applied with the frame modulating system, there poses a problem that there is power consumption due to the large number of changes of the column electrode waveform produced in one frame.
The problem to be resolved resides in reducing power consumption of a simple matrix type liquid crystal panel by restraining a number of changes in a waveform between a row electrode and a column electrode without deteriorating display quality.
In order to resolve the above-described problem, the invention is constituted by paying attention to the fact that according to a screen display of a simple matrix type liquid crystal panel driven by voltage averaging method, SA method or MLA method, gradation is not frequently and significantly changed by background color or commonly used display data.
That is, according to a first aspect of the invention, there is provided a method of driving a liquid crystal display panel holding a liquid crystal layer between a row electrode group and a column electrode group in accordance with given pixel data, wherein frame modulation is used as a gradation system and the order of scanning the row electrode group is selected discontinuously in conformity with the frame modulation pattern of background color or commonly used display data such that the change in a waveform of the column electrode group is minimized.
Further, according to a second aspect of the invention, there is provided the method of driving a liquid crystal display panel according to the first aspect wherein when the frame modulation is carried out at every row and rows are made ON and OFF alternately each electrode thereof at an intermediate gradation level, every other row thereof is selected.
Further, according to a third aspect of the invention, there is provided the method of driving a liquid crystal display panel according to the first aspect wherein when the frame modulation is carried out at each pixel and pixels are made ON and OFF alternately each pixel in the column direction and in the row direction at an intermediate gradation level thereof, every other pixel in each direction is selected.